1. Field of the Invention
This invention relates to valves, and more particularly, to a dual seal valve having a resilient primary seal and a metal-to-metal secondary seal. While a cylindrical plug valve is illustrated herein, the dual seal configuration is also applicable to ball valves, spherical plug valves, and other valve types with rotating valve elements.
2. Description of The Prior Art
Plug valves, and particularly plug valves of the tapered plug type, are well known. However, tapered plug valves are not well suited to high pressure conditions because the pressure tends to unseat the plug. To overcome this problem, firm seating of the plug is necessary which results in the disadvantage of greatly increased torque necessary to rotate the plug in its bore.
One way of overcoming this problem with plug valves is to use a cylindrical plug with an insert having a cylindrical inner surface and a tapered outer surface. This type of valve is disclosed in U.S. Pat. Nos. 2,813,695 to Stogner and 2,954,961 to Stogner et al., both assigned to the assignee of the present invention. In this kind of valve, an adjusting nut engages the insert wedging it between the valve body and the cylindrical plug, thus forcing metal-to-metal contact between the inner surface of the insert and the outer surface of the plug. An O-ring or similar seal provides sealing engagement between the outer surface of the insert and the valve body. Because the contact between the valve plug and the insert is along a cylindrical surface, the torque required to rotate the plug is considerably less than that of a tapered plug type valve. This insert type plug valve has been used successfully, but has some limitations in critical gas handling conditions because the metal-to-metal contact between the insert and cylindrical plug generally cannot be relied on to provide long-lasting, bubble-tight sealability as required by more stringent service specifications, such as those of the American Petroleum Institute. The present invention solves this problem by providing a resilient primary seal while still having a metal-to-metal secondary seal as a backup. The primary seal provides the bubble-tight sealability necessary in these more severe conditions.
A plug valve having elastomeric seals between inserts and a cylindrical plug is disclosed in U.S. Pat. No. 3,194,534 to Sachnik. However, the inserts in this valve are not tapered, and therefore, cannot provide the metal-to-metal seal as does the present invention.
Additionally, if a purely elastomeric material, such as rubber, were used for the seals in devices such as Sachnik, they are easily damaged as they pass over the openings in the plug. The valve in Sachnik contemplates the use of seal rings which are made of low friction and wear resistant plastic such as nylong, tetrafluorethylene (TFE), or other similar materials. Materials such as nylon and TFE do not have the tearing problems of rubber, but because they have no inherent resiliency, there are no internal stresses in the seal which can assure that it is maintained in good, sealing contact with the plug once the plug or seal has worn even slightly. The present invention solves this problem by providing a composite seal having a relatively hard, high lubricity element in contact with the valve element and an integrally molded, relatively soft, resilient elastomeric element which insures that the hard element is maintained in sealing engagement with the valve element.
U.S. Pat. No. 3,605,792 to Westbrook discloses a plug valve having a cylindrical plug and no inserts. Sealing between the plug and valve body is provided by a relatively resilient backup ring which fits in a groove in the body and resiliently biases a seal ring, made of a material such as TFE and also disposed in the groove in the body, against the plug. The backup ring and seal ring are separate elements, and therefore, the Westbrook valve is more difficult to assemble than the valve of the present invention using a composite ring. Further, Westbrook does not have any means for providing metal-to-metal sealing engagement against the cylindrical plug in addition to that provided by the seal rings. In other words, the Westbrook device is not a dual seal valve.
The present invention is an improvement over Westbrook because it uses a composite primary seal and has such metal-to-metal contact which provides a secondary seal in the event of failure of the primary seal. The metal-to-metal seal further acts to keep abrasives carried in the fluid handled by the valve from the primary seal.